Re: How do I check the MOSFET's in my stepper driver

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> Mariss Freimanis wrote:
>
> >1) Set your multimeter to Ohms.
> >2) Disconnect the motor and power supply from the drive.
> >3) Connect one meter lead to drive power supply ground.
> >4) Measure the resistance at each of the 4 outputs.
> >5) Move the lead from ground to drive + supply input.
> >6) Repeat (4)
> >
> >Transistors invariably fail shorted. If one or more of the 8
> >measurements you took show a short then the drive has 1 or more
> >damaged power transistors.
> >
> >
> I haven't seen this in 80 V stepper or servo drives, but in some

400 V

> inverter drives I designed some years ago (very similar full-bridge

switch

> circuits, though) I have on a few occasions had blown transistors

that were

> in the open condition AFTER a fault. Of course, these transistors

were open

> in more than the electrical sense! They sounded like a pistol

going

> off, indoors,
> and glowing pieces were embedded in the walls.
>
> (Yes, Mariss is correct above, the transistors shorted first, then

were

> exploded
> by shorting the energy in the filter caps.)
>
> Jon

Years back when I worked on large UPS installations in critical
applications (999 <911?> Fire, Police, Ambulance man mobilisation)
the expensive semiconductors were protected with germanium fuses that
would go o/c before the semiconductor switching element was damaged.
They were VASTLY expensive, and I suspect nowadays it is much cheaper
to let the MOSFET go pop rather than the fuse!

AWEM